It is often desirable to be able to measure the size and count the number of particles flowing in a fluid stream. For example, in medicine microembolisms in flowing blood are measured, and in the pharmaceutical industry contaminants in the form of solid particles need to be detected in drug products.
Pulse echo ultrasonic search units are well known, particularly in nondestructive testing systems; bursts or pulses emitted by such units are detected when reflected back from discontinuities. Pulse echo search units have also been used to measure particles in liquid. But these units directed the transmitted signal coaxially longitudinally of the conduit, with a focal point many times the diameter of the conduit in distance from the search unit's transmitting surface. Furthermore, they used flat or convex transmitting surfaces. My recently issued U.S. Pat. No. 4,112,773, hereby incorporated by reference, cites this prior art.
In measuring information using pulse echo search units, it is desirable to avoid generation of spurious returning acoustic signals. One source of these spurious signals is sound waves traveling along other than intended paths between the device and the target. Sensitivity of a unit is limited by pickup of such signals.